RFR: 8173585: Intrinsify StringLatin1.indexOf(char)

[Jason Tatton]

This is an implementation of the indexOf(char) intrinsic for StringLatin1 (1 byte encoded Strings). It is provided for
x86 and ARM64. The implementation is greatly inspired by the indexOf(char) intrinsic for StringUTF16. To incorporate it
I had to make a small change to StringLatin1.java (refactor of functionality to intrisified private method) as well as
code for C2.

Submitted to: hotspot-compiler-dev and core-libs-dev as this patch contains a change to hotspot and
java/lang/StringLatin1.java

https://bugs.openjdk.java.net/browse/JDK-8173585

Details of testing:
============
I have created a jtreg test “compiler/intrinsics/string/TestStringLatin1IndexOfChar” to cover this new intrinsic. Note
that, particularly for the x86 implementation of the intrinsic, the code path taken is dependent upon the length of the
input String. Hence the test has been designed to cover all these cases. In summary they are:

- A “short” string of < 16 characters.
- A SIMD String of 16 – 31 characters.
- A AVX2 SIMD String of 32 characters+.

Hardware used for testing:
-----------------------------

- Intel Xeon CPU E5-2680 (JVM did not recognize this as having AVX2 support) • Intel i7 processor (with AVX2 support).
- AWS Graviton 2 (ARM 64 processor).

I also ran; ‘run-test-tier1’ and ‘run-test-tier2’ for: x86_64 and aarch64.

Possible future enhancements:
====================
For the x86 implementation there may be two further improvements we can make in order to improve performance of both
the StringUTF16 and StringLatin1 indexOf(char) intrinsics:

1. Make use of AVX-512 instructions.
2. For “short” Strings (see below), I think it may be possible to modify the existing algorithm to still use SSE SIMD
instructions instead of a loop.

Benchmark results:
============
**Without** the new StringLatin1 indexOf(char) intrinsic:

| Benchmark  | Mode | Cnt | Score | Error | Units |
| ------------- | ------------- |------------- |------------- |------------- |------------- |
| IndexOfBenchmark.latin1_mixed_char | avgt | 5 | **26,389.129** | ± 182.581 | ns/op |
| IndexOfBenchmark.utf16_mixed_char | avgt | 5 | 17,885.383 | ± 435.933  | ns/op |


**With** the new StringLatin1 indexOf(char) intrinsic:

| Benchmark  | Mode | Cnt | Score | Error | Units |
| ------------- | ------------- |------------- |------------- |------------- |------------- |
| IndexOfBenchmark.latin1_mixed_char | avgt | 5 | **17,875.185** | ± 407.716  | ns/op |
| IndexOfBenchmark.utf16_mixed_char | avgt | 5 | 18,292.802 | ± 167.306  | ns/op |


The objective of the patch is to bring the performance of StringLatin1 indexOf(char) in line with StringUTF16
indexOf(char) for x86 and ARM64. We can see above that this has been achieved. Similar results were obtained when
running on ARM.

-------------

Commit messages:
 - 8173585: further whitespace changes required by jcheck
 - JDK-8173585 - whitespace changes required by jcheck
 - JDK-8173585

Changes: https://git.openjdk.java.net/jdk/pull/71/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=71&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8173585
  Stats: 522 lines in 15 files changed: 506 ins; 0 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/71.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/71/head:pull/71

PR: https://git.openjdk.java.net/jdk/pull/71